1. Field of the Invention
The present invention relates to a liquid crystal display apparatus. More specifically, the present invention relates to a liquid crystal display apparatus having improved viewing angle characteristics.
2. Description of the Related Art
Conventionally, liquid crystal display apparatuses using nematic liquid crystal display devices have been widely used as numerical value segment type liquid crystal display apparatuses such as clocks and calculators. In recent years, such apparatuses have been used for displays of word processors, computers, and navigation systems, and TVs.
In the above-mentioned liquid crystal display apparatus, an active element such as a thin film transistor (hereinafter, referred to as a "TFT") is generally formed for selectively driving a pixel electrode which applies a voltage to a liquid crystal layer. Furthermore, a color filter layer of red, green, blue, etc. is provided for the purpose of conducting a color display. As a display system, the following two liquid crystal display systems have been used, depending upon the twist angle of nematic liquid crystal to be used:
(a) An active drive type twisted nematic (TN) liquid crystal display system in which nematic liquid crystal molecules are aligned so as to be twisted by 90.degree..
(b) A multiplex drive type super twisted nemetic (STN) liquid crystal display system in which nematic liquid crystal molecules are aligned so as to be twisted by more than 90.degree..
The liquid crystal display devices driven by the above-mentioned two systems have problems such as viewing angle dependence and a small viewing angle. Therefore, various methods for solving these problems have been suggested.
For example, it is suggested in Japanese Laid-Open Patent Publication No. 57-186735 that each pixel be divided into two regions and the viewing angle characteristics of these regions made different, whereby each pixel is provided with a plurality of viewing angle characteristics. Furthermore, it is suggested in Japanese Laid-Open Patent Publication No. 7-248497 that liquid crystal molecules be aligned so as to be twisted by a plurality of twist angles in each pixel.
According to the method disclosed in Japanese Laid-Open Patent Publication No. 57-186735, each pixel has two viewing angle directions: a 12 o'clock direction and a 6 o'clock direction. In a liquid crystal panel having such viewing angle characteristics, viewing angle dependence exists in the vertical direction (i.e., 6-12 o'clock direction) and horizontal direction (i.e., 3-9 o'clock direction). In particular, the viewing angle dependence in the case where the display panel is seen in the horizontal direction (i.e., 3-9 o'clock direction) is almost the same as that in a TN liquid crystal panel to which a conventional alignment division method is not applied. More specifically, the problem of the viewing angle dependence is not solved even by applying the alignment division method. When a viewing angle is increased from the vertical direction (viewing angle of 0.degree.) to the horizontal direction to the display panel, black end white (contrast) of an image is inverted in the vicinity of a viewing angle of 30.degree. (gray-scale inversion phenomenon).
In the present specification, a "viewing angle" refers to an angle formed by a normal line to a display surface of a liquid crystal display device (liquid crystal panel) and a viewing angle direction (i.e., a direction in which a user sees the display surface: a line of sight). An "azimuth angle" refers to an angle formed by a component of a display surface in a viewing angle direction and a reference direction of the display surface. The reference direction is typically set in an upper direction (i.e., a 12 o'clock direction, for example, in the face of a clock) in the case where a user sees the display surface.
The wording "viewing angle characteristics" of a liquid crystal display apparatus refers to the dependence on the direction of a line of sight of a user in a wide sense. Specifically, it refers to the dependence of display characteristics on the above-mentioned "viewing angle" and "azimuth angle".
When the viewing angle is increased from 0.degree. in the vertical direction, averaged characteristics of the viewing angle characteristics in the 6 o'clock direction and in the 12 o'clock direction in the conventional TN liquid crystal panel can be obtained. Thus, the viewing angle characteristics obtained by increasing the viewing angle in the 12 o'clock direction become symmetric with respect to those obtained by increasing the viewing angle in the 6 o'clock direction. However, there is a limit to the extent contrast can be increased when the viewing angle is tilted down.
According to the methods described in the abovementioned publications, there is a limit to improvement of the viewing angle characteristics in the azimuth angle of 45.degree. from an absorption axis or a transmission axis of a polarizing plate.